1. Field of the Invention
This invention relates to a swivel fitting for marine mooring. More specifically, this invention relates to a swivel fitting which is readily protectable from corrosion and, when installed in a mooring line, readily allows swiveling connection to a plurality of mooring pendants or to an anchoring device.
2. Description of Related Art including Information Disclosed under .sctn..sctn.1.97 to 1.99
Traditional mooring practice employs various combinations of chain, shackle and swivels to connect by chain the eye of an anchor stock or mooring weight to a bouyant float and then, in turn, to a rope or cable pendant which makes the final attachment to the vessel. The vessel is free to move with wind and current. This movement induces twisting in the mooring chain, substantially decreasing the inherent strength of the chain and causing it to foul itself and reduce scope. In order to minimize these tendencies, moorings usually include one or more swivels to permit rotational movement of the vessel without causing the chain to develop a twist.
Moorings have been assembled (FIG. 1) from a combination of conventional rigging components, all of which have proved quite satisfactory for conventional dry land rigging applications, but readily susceptable to corrosion in marine use. A mooring is only as strong as its proverbial weakest link. Assuming that a mooring is assembled from components that have a similar strength rating --again common practice for reasons of economy--the swivels S are the weak links that most often lead to failure of the mooring. This failure can take one of two modes: either simple fracture failure of the swivel, or corrosion-induced seizing which prevents the intended swiveling action. Both failure modes can be directly traced to identifiable design weaknesses in the standard swivel when it is used in a corrosive marine environment.
Standard marine swivels are identified in FIGS. 1a and 1b. FIG. 1a shows a so-called "eye-and-eye" swivel. It comprises two rings, one above the other, the adjacent sides of the two rings being flattened and thickened and both apertured at A in alignment. A galvanized bolt extends through the two apertures and receives a nut to hold the assembly swivelly together.
FIG. 1b shows a so-called "jaw-and-eye" swivel. It comprises an upper ring or "eye" flattened and thickened and apertured like the upper ring in FIG. 1a. Completing the "jaw-and-eye" swivel is an inverted "Y"-shaped yoke, the stem of which is threaded and inserted through the aperture A in the "eye" and nutted.
Swivels intended for marine use are usually protected from corrosion by hot dip galvanizing. If the components are dipped prior to assembly, the aperture A in the "eye" is often drilled out to permit the bolt or shank of the jaw to rotate freely. If the swivel is assembled first and then galvanized, very little protective material will penetrate the aperture of the "eye", leaving both the aperture and the bolt or shank of the "jaw" under-protected from corrosion. In any case, the design intrinsicly predetermines that normal service wear will be concentrated within the failure zone, leading to an early erosion of any protective coating and encouraging rapid corrosion.
When a mooring is in continuous use, the accelerated deterioration in the failure zone will soon make the swivel the weakest link in the mooring, leading to a potential fracture failure in the case of heavy storm loading. If a mooring remains unused without a rotational load for sufficient time, the corrosion in the failure zone will cause the swivel to lose its rotational freedom, setting up failure through seizure when put back into service. This failure mode is particularly insidious because it cannot be avoided by simply using a larger, stronger swivel.
The present invention was conceived to avoid the weaknesses inherent in the conventional swivels while incorporating features that are especially suited for mooring applications. The present swivel fitting is designed to be used with one or more standard bow or anchor shackles or a shackle to the ring on the anchor stock. The shape of the present swivel fitting readily accepts hot dip galvanizing to full effect for maximum protection. The failure zone is more massively formed in the present swivel fitting than in conventional swivels, assuring to swivels comprising the present fitting a longer life to failure in a marine environment. Further, the standard shackle loosely encircles the swivel fitting neck so that corrosion cannot easily induce seizing.